1. Field of the Invention
The present invention relates to a socket for semiconductor devices that is placed between semiconductor devices and a burn-in board or the like in a burn-in test, etc.
2. Description of the Related Art
Most semiconductor packages employ electronic mechanism components called "sockets" placed between packages and boards to conduct testing. This type of socket for semiconductor devices usually has a spring portion known as a contact spring which provides three functions, namely, a function as a contact for semiconductor devices, a function as a spring, and a function as a terminal to be inserted in a board. Therefore, the contact spring used also as an electric circuit must be composed of a material having a low electrical resistance and also able to provide a spring load necessary when a semiconductor package is pressed against it. The spring portion of the socket for semiconductor devices is required to meet the following conditions: (1) With respect to a displacement of approximately 0.4 mm, the surface stress of the material stays within an elasticity limitation and the spring does not incur permanent set; (2) A contact load of about 10 to 15 grams is provided, or the load can be controlled freely or to a constant value with a simple parameter; and (3) The spring portion survives continuous use at 150 degrees Celsius and use of several thousands to several ten thousands of times in the case of a burn-in test.
A semiconductor package also serves to expand intervals between chip electrodes in units of microns to pitch intervals of a board in units of millimeters. Hence, conventional sockets for semiconductor devices can be easily manufactured by making contacts of millimeters in size by press molding or the like and installing them in the sockets.
In recent years, however, the trend toward a further reduced size and a higher density of electronic components is rapidly accelerating. Among BGAs, a package known as CSP in particular that has a pitch interval of 0.5 mm or less cannot be satisfactorily dealt with by a conventional method in which pins are manufactured one by one and installed in sockets, because only peripheral arrangement is possible by the conventional method. Hence, efforts have been made to develop a socket for semiconductor devices that is adaptable for pitches of higher-densities and permits easy manufacture.
For instance, as a contact portion, in addition to a contact probe type shown in FIG. 8A that has been used for long, an embedded-wire type, an implanted-conductive-particle type, cobra-structure type, and coil-spring type shown in FIGS. 8B through 8E, respectively, have been mainly used.
However, the contact probe type has extremely high manufacturing cost although it features good spring characteristics, thus limiting its application range. The embedded-wire type has been posing a problem in that a displacement required as a spring cannot be obtained; a slight displacement causes a sudden rise in load, leading to damage to a terminal of a semiconductor package. The implanted-conductive-particle type has been inconvenient in that it exhibits a high electrical contact resistance in addition to the same problem as that of the embedded-wire type. The cobra-structure type has been posing a problem in that it is no suited for burn-in tests because it has a small wire diameter and provides an insufficient spring pressure accordingly. The coil-spring type has been presenting a problem in that it exhibits high electrical resistance because of its long total length, making itself electrically disadvantageous.
In conventional sockets, press-molded springs are usually employed to secure contact between semiconductor device terminals and socket contact pins. However, the load exerted by a spring increases as a displacement thereof increases. Hence, if loads applied to semiconductor device terminals vary due to variations in size of semiconductor device terminals, then poor contact takes place in a terminal with a smaller load applied thereto, while a terminal of a solder ball or the like deforms if a larger load is applied thereto. This has been making it difficult to acquire accurate test results especially in burn-in tests. Furthermore, a press-molded spring has been presenting a shortcoming of limited adaptability for terminals having high-density pitches because the press-molded spring requires an area or volume for each pin.
In addition, the conventional type socket merely has a contact that fits a terminal pitch of a semiconductor package, and does not have a space required for accommodating the spring and the lead that have been integrally designed and press-molded; therefore, the conventional type socket cannot have a function for extension to a board. For this reason, in order to mount the socket on the board, it is necessary to accomplish fan-out by using a separate multilayer board for routing wires, posing a problem in that a burn-in test costs excessively high.
Furthermore, a terminal pitch used to be 2.54 mm in the past, whereas a present dominant terminal pitch is 1.27 mm. It is expected that the trend toward a higher density will reach a terminal pitch of about 0.5 mm to about 0.3 mm as the sizes of semiconductor packages are reduced in the future. In this case, the method employing a conventional multilayer board will further add to the cost of burn-in tests.
In addition, the spring portion of the semiconductor device socket is formed by punching a complicated shape by a press or the like, or formed by bending after punching with a press. The pitch of a fan-out portion is usually widened by bending. In most cases, therefore, producing semiconductor device sockets has been requiring efforts and high cost. The pitches of the CSP terminals, in particular, are small, presenting a problem in that producing burn-in sockets compatible with the CSP terminals requires still higher cost and more efforts because of limited machining accuracy.